The present invention concerns navigational aid devices and relates more particularly to the detection and warning of collision risks in high sea and coastal navigation.
The introduction on board ships of all categories of radars has enabled navigators to assess the risks of collision with the obstacles which surround them.
Moreover maritime traffic surveillance stations have recently been created in many countries. Among other missions, these radar-equipped stations maintain a watch to prevent collision risks between ships.
If, on the radar screen carried by own ship A, the successive echos corresponding to a single fixed or moving target B are plotted, one can know the relative speed V.sub.R of this target and thus assess:
the distance of closest point of approach, commonly designated in maritime parlance by the abbreviation DCPA, PA1 the time of closest approach TCA, PA1 the position of own ship A and that of the target B at the closest approach. PA1 the risk of collision with that obstacle, PA1 the value of the distance of closest point of approach, PA1 the time of closest point of approach, PA1 the position of own ship and that of the obstacle at closest point of approach, PA1 heading and speed manoeuvres that he can make to pass at a safety distance from the obstacle, which he fixes in advance, without inasmuch risking a collision with another obstacle, PA1 the effect of the manoeuvre that he will have decided to make.
For some years, anti-collision systems based on calculators enable automatic plotting of targets surrounding own ship, and providing navigational aids to warn of collisions.
The principle of these systems consists in calculating the speed and heading of the targets which are within radar range, deducing by calculation the values of DCPA and TCA, signalling alarms to the navigator if these values become less than predetermined safely threshold values. Cartain more sophisticated systems can give indications of changes of heading to be effected by the ship to avoid a threatening obstacle and in certain cases indication of the effect of a modification of the speed.
However these systems make use of very long calculations and do not give simultaneously an overall view of the manoeuvre possibilities of own ship in heading and speed.